(a) Multiink systems--A new class of photographic inkjet printers uses multiple inks. For instance instead of the traditional four inks (cyan, magenta, yellow and black, symbolized CMYK) a printer may carry six inks, including two densities, dilutions or shades of cyan and magenta inks.
Neither of the two new cyan inks is exactly the same density etc. as the traditional cyan C; and neither magenta ink is the same as traditional magenta M. To distinguish symbolically between the light and dark cyan and magenta inks in such a printer, this document uses the notations C.sub.L, C.sub.D, M.sub.L and M.sub.D respectively. Thus the full complement of available inks is denoted C.sub.L C.sub.D M.sub.L M.sub.D YK.
The new generation of multiink machines is also distinctive in (1) permitting greater numbers of inkdrops per pixel as a matter of routine, and (2) seeking to perform halftoning (particularly error diffusion) based upon a full color specification or vector, rather than channel by channel.
In previous inkjet printers, generally the number of inkdrops heretofore employed at each pixel is very constrained: at first, two primary-ink drops were used only to form secondary colors (e. g. red, green and blue RGB), and more recently limited usage of two or three drops on a single pixel has been permitted for better saturation or more consistent effects between colors (particularly on nonabsorbent printing media such as transparencies for overhead projection, and other special-use plastic media).
As to halftoning, heretofore in most systems input color values if received or generated in terms of RGB are first converted to CMY; the CMY data are converted to CMYK, and each of the four color signals are halftoned separately to establish amounts of the corresponding four inks to print for each pixel. A partial relaxation of this regimen is developed in the above-mentioned patents of Best and Dillinger, Perumal and Dillinger, Motta and Dispoto, and Sullivan et al.
The new systems to a great extent cast aside both these earlier constraints. Even though the number of visually distinguishable colors that can be made with these systems is still several orders of magnitude smaller than available with a device (such as a cathode ray tube) that has nearly continuous control in three color dimensions, nevertheless the new ink technology opens enormous new capabilities for production of both subtle and rich color effects at astonishingly low cost and with remarkable speed. Halftoning and color control, however, become monumentally more complex and challenging.
Optimizing ink usage per pixel, constraining colors to a maximum number of drops per pixel, making fullest use of the capability of multiink, multidrop systems to produce a reasonably complete gamut--with smooth-appearing color gradations--and minimizing graininess are all new goals. In addition, developing an algorithm that is amenable to a hardware implementation is important for fully adequate speed or image throughput.
(b) Error diffusion--This topic has been extensively elaborated in both the nonpatent and patent literature, as for instance in the above-mentioned references of Ulichney, Heitsch (a four-color implementation), Sullivan et al., and Motta et al. Furthermore the application of the present invention to error diffusion in multiink, multidrop systems is described and explained in our own above-mentioned concurrently filed patent document "Device State Error Diffusion Technique for Halftoning".
That document shows that the present invention enables performance of error diffusion both very efficiently and, if preferred, wholly in a full-color space. Even propagated "error" signals can, if desired, be distributed out of that process to only the basic three-dimensional color coordinates (not all of the multiple ink channels). In this document we therefore refrain from dwelling on the characteristics or operation of error-diffusion systems.
(c) Systems based on full-palette measurement--It is noteworthy that heretofore efforts to deal with the great new expanse of capabilities in multiink, multidrop systems have focused upon characterizing the entire printer palette systematically at the outset through relatively exhaustive photometric work. This is particularly so, for instance, with respect to Motta, mentioned above--which also emphasizes performance of such characterization in a perceptual space as for instance through conversion of initial color data from RGB to the familiar CIELAB coordinates.
We by no means intend to disparage the very fine work of Motta and Dispoto; their guidelines appear insightful, reasonable and all but guaranteed to yield both excellent color performance, in terms of image quality, and a thorough understanding of that performance. What has been unexpected and somewhat mystifying about that approach, however, is that the relatively great initial investment in a careful measurement phase has not appeared to be repaid in terms of either ultimate quality or comprehension.
In our efforts to apply such a recipe to a pragmatic regime of multiink, multidrop color printing, much to our surprise we achieved only relatively poor gradations, leading to contouring, and even slight errors in sequencing. These errors were especially troublesome in attempting to reproduce slow continuous gradations in an input image.
Initially we found these effects baffling. Only as a result of our own work as set forth in other parts of this document have we begun to understand how it is that such insidious problems manage to creep into a seemingly failure-proof and rigorously analytical, systematic approach.
Moreover, in Motta all color corrections or conversions are built into device-state tables, leading as a practical matter to inseparability of color matching from error diffusion. While this consolidation is in principle very efficient and useful, it introduces undesirable complexity--and in particular delay--into the common task of printing objects of various different types and characteristics within a single page.
(d) Other earlier efforts of present inventors--In addition to the work discussed above we have also personally explored several other measurement-based strategies. In one such effort, we developed measurements in CIELAB of three to four hundred tiles, manipulated the results to express the measurements in terms of an XYZ-like space, and then further manipulated those values to reach CMY coordinates. Results for this work were analogous to the problems discussed above, as were still others in an exploration based on the hue-plus-gray or hnk space.
Another program of particular interest explored independent-channel error diffusion in a six-ink proportional system: this method worked rather well in terms of gradation smoothness and monotonicity--and hence absence of contouring--as well as speed. Because of the independent channels, however, liquid control was lacking.
As will be detailed below, we have found that control and per-pixel balance of liquid loading exert a remarkably great influence on granularity, smoothness of gradations, and even colorimetric accuracy. These effects are in addition, of course, to the more widely and historically recognized gross effects of excessive or irregular liquid load on printing-medium drying time, local distortion, blocking and offset. We have therefore rejected the appeal of independent-channel processing.
It will be understood that much of the foregoing discussion deals with work done heretofore, by the present inventors and by others, that is not prior art with respect to the present invention.
(e) Conclusion--Heretofore such limitations of stratagems for establishing and controlling printer palettes have continued to impede achievement of uniformly excellent multiink, multidrop inkjet printing--at high throughput--on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.